Confirmation via the Continuum-Fitting Method That the Spin of the Black Hole in Cygnus X-1 Is Extreme

In Gou et al., we reported that the black hole primary in the X-ray binary Cygnus X-1 is a near-extreme Kerr black hole with a spin parameter a * > 0.95 (3σ). We confirm this result while setting a new and more stringent limit: a * > 0.983 at the 3σ (99.7%) confidence level. The earlier work, which was based on an analysis of all three useful spectra that were then available, was possibly biased by the presence in these spectra of a relatively strong Compton power-law component: the fraction of the thermal seed photons scattered into the power law was f s = 23%–31%, while the upper limit for reliable application of the continuum-fitting method is f s ≲ 25%. We have subsequently obtained six additional spectra of Cygnus X-1 suitable for the measurement of spin. Five of these spectra are of high quality with f s in the range 10%–19%, a regime where the continuum-fitting method has been shown to deliver reliable results. Individually, the six spectra give lower limits on the spin parameter that range from a * > 0.95 to a * > 0.98, allowing us to conservatively conclude that the spin of the black hole is a * > 0.983 (3σ).